The present invention relates to a spontaneous light emitting display device using an active matrix spontaneous light emitting element (a spontaneous light emission light emitting element) and having an excellent luminance uniformity and a lowered power consumption.
As a spontaneous light emitting display device, for example, an organic EL display using an organic EL (a spontaneous light emitting light emitting element) for a display panel has reached a practical level. The organic EL will be described below. Since the organic EL display has excellent features which cannot be obtained by a liquid crystal display, for example, spontaneous light emission, high-speed response and wide angle of view, it is expected to be a flat panel display which can produce clear characters and graphic images in a dynamic image display. The organic EL display can be operated in a passive matrix (PM mode) and an active matrix (AM mode) depending on a driving method.
The PM type is provided with a driving circuit outside of an organic EL panel and the structure of the organic EL panel itself can be therefore simplified and cost can be reduced. At present, the PM type organic EL panel is manufactured as a product and is used for a vehicle or a mobile telephone. The organic EL is a current driving element. In order to eliminate variation in luminance of the organic EL panel, therefore, it is necessary to cause a current flowing to each light emitting pixel to have an equal magnitude. However, it is hard to have an equal current, and furthermore, to reduce power consumption due to the following problems A to C.
A. In order to cause luminance of all pixels to be uniform, the current flowing to each of the pixels is to be equal. For this reason, it is necessary to cause one of the positive and negative electrodes of each pixel to act as a constant current source. In order to operate the electrode as the constant current source, however, the driving voltage of a matrix electrode on the other side is to be increased such that a voltage drop caused by the resistance component of a bus line has no influence. Consequently, a power consumption is increased. In the case in which a driving voltage cannot be increased sufficiently, a voltage drop corresponding to a bus line length to reach each pixel influences a current amount for light emission. More specifically, the constant current source is not obtained so that a variation in luminance is caused.
B. In order to obtain a predetermined surface luminance, the PM type needs to emit a light with an N-fold instantaneous luminance if the number of scanning lines of the display panel is N. Since a current flowing to the pixel is usually proportional to a light emission luminance, the current to flow becomes N-fold. Since the organic EL has such a feature that a light emission efficiency is reduced if a current to flow is increased, however, an N-fold pixel current or more is required for obtaining the predetermined surface luminance. Thus, the power consumption is increased if the number N of the scanning lines is increased. This problem increasingly promotes the problem A.
C. Since the organic EL panel has a surface structure, a capacitive load is connected to each element in parallel as an equivalent circuit. When a pixel current is increased or the number of pixels is increased so that a repetitive frequency is increased, the magnitude of a charging and discharging current to flow to the capacitive load is made great so that a power consumption is increased. Due to the problem B, the power consumption of the capacitive load is considerably increased in the PM type. Due to the above problem, the PM type which is currently manufactured as a product has a screen size of several inches or less and a pixel number of 10,000-pixel level.
In the AM type organic EL panel, the above problems can be improved. In the above problem A, the AM type has a TFT driving circuit provided in each pixel. Therefore, it is not necessary to cause a large current to flow instantaneously. As a result, a voltage drop caused by a bus line in the above problem A is decreased and an applied voltage can be reduced. Consequently, the power consumption can be reduced more considerably than that of the PM type.
Since the applied voltage can be reduced, a slightly high applied voltage is simply set so that a voltage drop corresponding to a bus line length to each pixel does not influence on a pixel current amount. Consequently, an uniform luminance can be obtained. In the above problem B, the AM type has a TFT driving circuit provided in each pixel. Therefore, it is sufficient that a small pixel current always flows irrespective of the number N of the scanning lines. Therefore, there can be avoided increase of power consumption due to reduction in light emission efficiency with an increase in a pixel current. In the problem C, since the AM type has the TFT driving circuit provided in each pixel, it is sufficient that a small pixel current flows irrespective of the number N of the scanning lines. Therefore, a charging and discharging current flowing to the capacitive load can be reduced. Consequently, the power consumption can be reduced. Thus, the AM type organic EL panel can reduce a variation in luminance and a power consumption. However, the AM type has the following great drawback. More specifically, it is hard to fabricate a driving element having a uniform characteristic over the whole organic EL panel area. As a result, a current value flowing to each pixel is different so that a luminance is varied. FIG. 12 shows a driving circuit for causing a pixel in a conventional AM type organic EL panel to emit a light which has been described in Japanese Patent Publication No. 2784615, for example.
An operation will be described below with reference to FIG. 12.
Reference numeral 80 denotes a FET constituted by an N channel type and is operated as a switching element. Reference numeral 81 denotes an FET constituted by a P channel and is operated as a driving element. The FETs 80 and 81 are formed of low temperature polysilicon. A capacitor 82 is a capacitive load connected to the drain terminal of the FET 80. An organic EL element 83 to be an light emitting pixel is connected to the drain terminal of the FET 81. The drain terminal of the FET 80 is connected to the gate terminal of the FET 81. A scanning signal is applied from a scanning line 84 to the gate terminal of the FET 80. An image signal is applied from a data line 85 to the source terminal of the FET 80. Reference numeral 86 denotes a voltage supply line for supplying a voltage to the organic EL element 83. First of all, a scanning signal is applied to the gate terminal of the FET 80. At this time, when an image signal is applied at a predetermined voltage to the source terminal, the capacitor 82 of the drain terminal of the FET 80 is held to have a voltage level V1 corresponding to the magnitude of an image signal. If the magnitude of the voltage level V1 (shown in FIG. 13) held to have the gate voltage of the FET 81 is enough for causing a drain current to flow, a current corresponding to the magnitude of the voltage level V1 flows to the drain of the FET 81. The drain current becomes a light emitting current for a pixel. The luminance of the pixel is proportional to the magnitude of the light emitting current.
FIG. 13 is a characteristic chart for explaining the generation of a luminance variation in a pixel when a light emission is carried out by such an operation. The characteristic chart shows the relationship between a gatexe2x80x94source voltage and a drain current of the FET 81. In the case in which the FET 80 and the FET 81 are formed of the low temperature polysilicon, it is hard to obtain an FET having the same characteristic over the whole display panel area in respect of manufacturing method of a low temperature polysilicon. For example, each of the FET 80 and the FET 81 has a variation in characteristic shown in FIG. 13. When the voltage level V1 is applied to the FET 81 having such a characteristic, the magnitude of the drain current is varied in a range of Ia to Ib. Since the organic EL emits a light with a luminance which is proportional to the magnitude of the current, the characteristic of the FET 81 represents a variation in light emission luminance. In particular, the variation in characteristic shown in FIG. 13 cannot prevent the generation of the luminance variation in a method of modulating a luminance in an analog amount, that is, a method of controlling a light emission luminance with the magnitude of the voltage level V1. In a digital luminance control method for controlling a luminance at a level in which the voltage level V1 shown in FIG. 14 always has a constant value, therefore, a level in which a current is saturated is used. Consequently, it is possible to prevent a luminance variation generated in an analog luminance control method.
In the case of a characteristic having the relationship between the gatexe2x80x94source voltage and the drain current of the FET 81 shown in FIG. 15, however, a saturation current is not equal. Also in the digital luminance control method, therefore, a luminance variation is generated. Thus, it is difficult to prevent the luminance variation by the characteristic variation of a driving element in the conventional driving circuit.
FIG. 16 shows a driving circuit described in xe2x80x9cActive Matrix OELD Displays with Po-SiTFT. The 10th International Workshop on Inorganic and OEL. P. 347 to P. 356xe2x80x9d as a conventional example in which the characteristic variation of the driving element is improved. In this conventional example, the FET 81 to be a driving element shown in FIG. 8 is set to be an FET 81A and an FET 81B, and there has been disclosed a structure in which these FETs are connected in parallel with each other to average the characteristic variation. Also in such a structure, it is very hard to prevent the generation of the characteristic variation in the driving element.
In the conventional spontaneous light emitting type display device, there has not been solved the problem in which the light emission luminance of a light emitting element constituting a pixel caused by the characteristic variation of the driving element is varied as described above.
The present invention solves the above-mentioned problems and provides a spontaneous light emitting type display device having no luminance variation in a pixel and a low power consumption.
A spontaneous light emitting type display device according to a first aspect of the present invention comprises a plurality of light emitting pixels arranged in a matrix state, a photoelectric converting portion provided for each light emitting pixel and serving to receive a light emitted from the light emitting pixel, and a control circuit for controlling a current flowing to the light emitting pixel by using a voltage obtained from the photoelectric converting portion.
According to such a structure, even if a variation in current is generated in a driving element, it is possible to suppress a luminance variation in the light emitting pixel.
A spontaneous light emitting type display device according to a second aspect of the present invention is characterized in that, in the spontaneous light emitting type display device according to the first aspect of the present invention, the control circuit includes a voltage and current converting portion and a current adding and subtracting portion.
According to such a structure, even if a variation in current is generated in a driving element, it is possible to suppress a luminance variation in the light emitting pixel.
A spontaneous light emitting type display device according to a third aspect of the present invention is characterized in that, in the spontaneous light emitting type display device according to the first aspect of the present invention, the device includes a voltage converting portion provided on an output side of the photoelectric converting portion, and means for controlling a current flowing to the light emitting pixel by using a voltage obtained from the voltage converting portion.
According to such a structure, it is possible to suppress a fluctuation in the luminance of the light emitting element due to a change in the conversion gain of the photoelectric converting portion.
A spontaneous light emitting type display device according to a fourth aspect of the present invention is characterized in that, in the spontaneous light emitting type display device according to the third aspect of the present invention, an output voltage of a voltage converting portion is controlled such that magnitudes of the output voltage and an image signal for controlling the light emitting pixel correspond to each other with one to one.
According to such a structure, it is possible to suppress a fluctuation in the luminance of the light emitting element due to a change in the conversion gain of the photoelectric converting portion.